1. Field of the Invention
The present invention relates to miniature auto-focusing lens devices, and more particularly, to a lens device which is furnished with a movable lens set and a voice coil motor in order to perform focusing operation.
2. Description of the Prior Art
In recent years, there is a trend toward handheld electronic devices capable of digital-photo shooting and digital-video recording. Handheld electronic devices of this kind are usually compact and operated by light-duty batteries; hence, size and power consumption are two essential factors in designing lens modules for use with the handheld electronic devices.
Digital cameras or digital camcorders, which are solely intended for photo shooting or video recording, can work quite well when equipped with a lens module capable of optical zooming and focusing even though the lens module is bulky. However, those handheld electronic devices whose principal function is not photo shooting or video recording, such as cell phones, notebook computers, or personal digital assistants (PDAs), are allowed with little room for accommodating the lens module. In general, the overall volume of miniature lens modules for use with handheld electronic devices, such as cell phones, notebook computers, or personal digital assistants (PDAs), is always less than 1.5 cm*1.5 cm*1.5 cm (e.g., width*length*height). Owing to the stringent volumetric restriction, nearly all the miniature lens modules are merely capable of optical focusing but seldom capable of both optical focusing and optical zooming. In other words, a conventional optical zooming mechanism for wide use with digital cameras or digital camcorders does not apply to the miniature lens modules, whether the conventional optical zooming mechanism works by an optical means or a mechanical means.
Commercially available miniature lens modules drive a lens to perform auto-focusing mostly by an electromagnetic technique commonly known as VCM (Voice Coil Motor). Referring to FIG. 1, a conventional miniature lens module 9 comprises a casing 91, a lens carrier 92, a plurality of magnets 93, a drive coil 94, a small-size first lens 95, and a large-size second lens 96. The lens carrier 92 is disposed in a receiving space centrally provided in the casing 91. The lens carrier 92 is peripherally provided with the drive coil 94 corresponding in position to the magnets 93 attached to the casing 91 and equidistantly spaced apart. The lens carrier 92 is provided therein with the first lens 95 and the second lens 96 corresponding in position thereto.
The drive coil 94 is supplied with preset electric currents transmitted in different directions to thereby produce electromagnetic fields in different directions. Hence, the magnets 93 fixed to the lens carrier 92 produce repulsive forces or attractive forces under which the lens carrier 92 undergoes axial displacement in the receiving space of the casing 91 such that the first lens 95 and the second lens 96 move synchronously inside the lens carrier 92 to thereby effectuate optical focusing.
The second lens 96 corresponds in position to an image sensing module 81 provided on a substrate 8. For reasons related to optical imaging, the second lens 96 usually outstrips the first lens 95 in area, and both the first lens 95 and the second lens 96 are confined to the lens carrier 92. During a focusing operation, in addition to the lens carrier 92, both the first and second lens 95, 96 are driven by the drive coil 94 and electromagnetic forces produced by the magnets 93. In other words, the total magnitude of the forces required to move the lens carrier 92 is relatively great, and thus the size and weight of the drive coil 94 encircling the lens carrier 92 and the magnets 93 corresponding in position to the drive coil 94 are relatively great. Hence, not only is a large electric current supplied to the conventional miniature lens module 9 in order to generate power sufficient for a focusing operation, but it is impossible to miniaturize the conventional miniature lens module 9. Also, dust and microparticles which are produced by friction in the course of assembly of the conventional miniature lens module 9 or originate from an external source are likely to accumulate in the vicinity of the image sensing module 81 to the detriment of the image quality thereof. Accordingly, there is still room for improvement in the conventional miniature lens module 9.